Late Cambrian geomagnetic instability after the onset of inner core nucleation

The Ediacaran Period marks a pivotal time in geodynamo evolution when the geomagnetic field is thought to approach the weak state where kinetic energy exceeds magnetic energy, as manifested by an extremely high frequency of polarity reversals, high secular variation, and an ultralow dipole field strength. However, how the geodynamo transitioned from this state into one with more stable field behavior is unknown. Here, we address this issue through a high-resolution magnetostratigraphic investigation of the ~494.5 million-year-old Jiangshanian Global Standard Stratotype and Point (GSSP) section in South China. Our paleomagnetic results document zones with rapid reversals, stable polarity and a ~80 thousand-year-long interval without a geocentric axial dipole field. From these changes, we suggest that for most of the Cambrian, the solid inner core had not yet grown to a size sufficiently large to stabilize the geodynamo. This unusual field behavior can explain paleomagnetic data used to define paradoxical true polar wander, supporting instead the rotational stability of the solid Earth during the great radiation of life in the Cambrian.

I suggest minor changes before it is considered for publication.
- Figure 3c and 3d : at the top of these two figures, I suggest to write "ChRM Declination" and "ChRM Inclination" -Figure 3 I see that the sampling resolution within Zone I is different from Zone II and III. How come? Is this perhaps due to an inappropriate lithology for the portable rock drill? I am asking this because this is the range with many inversions and some of these are defined only on a single sample... Personally I am skeptical when I see single samples with different inclination from the surrounding samples but this is a particular context and so I would just like to understand better. Finally, I see that for paleomagnetic data analysis you used Puffinplot. Please note that the most updated version is the following: Lurcock, P.C., Florindo, F., 2019. New developments in the Puffin Plot paleomagnetic data analysis program. Geochem. Geophys. Geosyst. 20, 5578-5587. https://doi. org/10.1029/2019GC008537 Reviewer #2 (Remarks to the Author):

General Comment
This paper presents an analysis of new sedimentary data from China in the early Cambrian. They identify periods of anomalous geomagnetic instability and frequent reversals, consistent with observations from earlier Precambrian-Cambrian boundary and Ediacaran. The authors hypothesize that the core was in an anomalous state at this time because the inner core was still small, which agrees with some models.
My only general comment is regarding the connection the authors draw between the paleomagnetic data and the previously published ideas of true polar wander during this time period. The authors make a valid point that paleogeographic models should acknowledge the possibility of true dipole wander (that is non-GAD behavior) and should encourage more investigation into the paleomagnetic signature of a non-GAD field vs TPW. The title of the paper draws undue attention to this aspect with the phrase "and its bearing on Earth's rotational stability". This rather vague phrase is somewhat misleading. In my opinion the paper would be better received by focusing on its own results: that the early to mid Ordovician geomagnetic field shows directional anomalies similar to those found in the Cambrian and late Neoproterozoic. This alone is an important discovery. The connection to TPW is more of a speculation at this point, albeit an important one.

Detailed Comments
Lines 124-126: What about the data implies magnetite? What about the data implies PSD or SD/MD? Lines 170-171: What "lithology and sedimentary features" are you referring to? What features would indicate an abnormal sedimentary process if one occurred?
Lines 172-173: What about the magnetic data indicates that the recording ability of the magnetic carriers does not change? Please be more specific. : "… may be a characteristic of the core during its first ~70 Myr of growth…" By "core" do you mean "inner core"? Lines 205-207: "it is important to emphasize that our hypothesis linking core size and field stability generally applies for all plausible ICN onset ages equal to or older than the Ediacaran Period." The data presented here is from the Cambrian, so how does it relate to the geomagnetic field of the Ediacaran or older? Please clarify. Lines 222-223: "Therefore, our observations of field instability are consistent with a rotationally stable Cambrian Earth." This seems like an overreach at this point. Sure, non-GAD field should be acknowledged, if not accounted for, in TPW models. But how do you do that? A more careful investigation into the paleomagnetic signature of non-GAD vs TPW seems necessary to even address this question. The inner core nucleation and true polar wander are two hotly debated topics in paleomagnetic studies. Recent paleomagnetic studies consistently indicate inner core nucleation in the Ediacaran Period characterized by extremely high frequency of polarity reversals, high secular variation, and ultralow dipole field strength. How the geodynamo transitioned from this state prior to and during inner core nucleation into a stable geocentric axial dipole state is fascinating. Another intriguing and controversial question of the Ediacaran and Cambrian periods is the observed polar wander, which is usually interpreted to indicate rapid rotation of the entire solid Earth relative to the spin axis. This, if true, could have induced dramatic environmental and climate changes affecting animal evolution. The reversal frequency of the geomagnetic field determined from magnetostratigraphic studies of well-dated and continuously deposited sedimentary strata is a valuable parameter to estimate the stability of the geodynamo. In this research, Li et al. present a high-resolution magnetostratigraphic investigation of the Jiangshanian GSSP section in South China, which provide the opportunity to decipher such puzzles.
Compared to other magnetostratigraphic studies on this topic, the age of the Jiangshanian GSSP section is well constrained to ~494.5 Ma, extending to the late Cambrian. The reported paleomagnetic and rock magnetic data reported in this research are of high quality, the analyses of these data are robust, and the results are well presented. The authors identify hyper reversals interval of 109 kyrs, stable reversed polarity interval of ~31 kyrs, a ~80 thousand-year-long interval without a geocentric axial dipole field, and a following stable normal polarity interval of >590 kyrs upsection. These findings suggest that the inner core has not yet grown to a size sufficiently large to stabilize the geodynamo in Cambrian and that such unusual field behavior can explain paleomagnetic data used to define paradoxical true polar wander. This work will be a great contribution to the paleomagnetic study of the inner core evolution and true polar wander, and will also interest a broad community of geoscientists working on geodynamo, paleontology, plate tectonics, and environmental changes. I fully agree with the publication of this work in Nature Communications.
My major suggestion to the manuscript is limited to strengthen the rock magnetic part and provide SEM/TEM observations to firmly support the primary origin of the HTC isolated from these carbonates. The preservation of a primary remanence magnetization in these carbonates is the base of this research. Although I agree that positive fold and reversals tests presented in this research strongly suggest a primary origin of the HTC, a complete and convincing demonstration of this requires robust rock magnetic and petrographic evidences. Actually, the wasp-waisted hysteresis loops and distribution of the ratios of hysteresis parameters in or near the remagnetization region of carbonates in the Day plot show risk of remagnetization of these rocks, which reminds that the carrier(s) and acquisition processes of remanence (LTC, ITC, HTC) within these rocks should be carefully discussed. To argue that remagnetization is restricted to the LTC and ITC and that the HTC is of primary origin, additional experiments might be required. I would suggest the authors to conduct low-temperature susceptibility or magnetization experiments to see if Verwey transition can be identified. Verwey transition at 120 K is usually smeared in remagnetized carbonate, which is dominated by oxidized authigenic magnetite (Jackson and Swanson-Hysell, 2012). If the magnetic carrier of the HTC is detrital (titano)magnetite or biogenic magnetite, which can carry a primary remanence, Verwey transition should be detected. Furthermore, SEM or TEM observations can verify the existence of detrital or biogenetic magnetite and authigenic magnetic minerals in these rocks. I also notice that the authors applied TEM observations on magnetic extracts from carbonates rocks of the lower and middle Cambrian strata in this region (Jiao et al., 2018), but this has not been applied on the upper Cambrian strata. SEM/TEM observations, when coupled with complete rock magnetic experiments, will greatly help to understand the carriers and origins of remanence in carbonates from the upper Cambrian strata shown in this manuscript.
Below are some minor suggestions.
Line 121: The temperature range of the LTC, ITC and HTC should be defined here.
Line 123: The authors argued for a Triassic remagnetization for the ITC carried by pyrrhotite in Jiao et al. (2018). However, this point has not been discussed in this manuscript. Is there anything different in rock magnetism for rocks studied here and Jiao et al. (2018)? A low-temperature magnetization experiment may also help to identify pyrrhotite with Besnus transition at 32 K in these rocks. Actually, the y axis magnetization with applied field of 0.5 T in Extended Data Fig. 3a show the sign of pyrrhotite. This should be carefully discussed in the manuscript.
Line 179: The NRM is dominated by secondary remagnetization signals, I wonder if it is meaningful to use it for such an argument. Maybe the intensity of the HTCs is more suitable. If there are other secondary magnetic carriers, then the stratigraphic changes of values of ARM, SIRM and their ratios in Fig. 2 should be reevaluated.
Line 184: Should inclination shallowing be considered here?
Extended data Fig. S: It seems that only 8.70 m specimen contains quite some hematite, other specimens have little concentration of hematite as indicated by saturation of the remanence before 0.5 T. For that reason, 8.70 m specimen is not suitable to plot in the Day plot because Day plot is for (titano)magnetite, and that wasp-waisted hysteresis loops of most samples do not indicate mixture of magnetite with hematite. Instead, these loops are very similar to the shape of remagnetized carbonate rocks (Jackson and Swanson-Hysell, 2012). This problem has to be addressed. Compared to other magnetostratigraphic studies on this topic, the age of the Jiangshanian GSSP section is well constrained to ~494.5 Ma, extending to the late Cambrian. The reported paleomagnetic and rock magnetic data reported in this research are of high quality, the analyses of these data are robust, and the results are well presented. The authors identify hyper reversals interval of 109 kyrs, stable reversed polarity interval of ~31 kyrs, a ~80 thousand-year-long interval without a geocentric axial dipole field, and a following stable normal polarity interval of >590 kyrs upsection. These findings suggest that the inner core has not yet grown to a size sufficiently large to stabilize the geodynamo in Cambrian and that such unusual field behavior can explain paleomagnetic data used to define paradoxical true polar wander. This work will be a great contribution to the paleomagnetic study of the inner core evolution and true polar wander, and will also interest a broad community of geoscientists working on geodynamo, paleontology, plate tectonics, and environmental changes. I fully agree with the publication of this work in Nature Communications.

References
Response 1: We thank the reviewer for this assessment. Comment 2: My major suggestion to the manuscript is limited to strengthen the rock magnetic part and provide SEM/TEM observations to firmly support the primary origin of the HTC isolated from these carbonates. The preservation of a primary remanence magnetization in these carbonates is the base of this research. Although I agree that positive fold and reversals tests presented in this research strongly suggest a primary origin of the HTC, a complete and convincing demonstration of this requires robust rock magnetic and petrographic evidences. Actually, the wasp-waisted hysteresis loops and distribution of the ratios of hysteresis parameters in or near the remagnetization region of carbonates in the Day plot show risk of remagnetization of these rocks, which reminds that the carrier(s) and acquisition processes of remanence (LTC, ITC, HTC) within these rocks should be carefully discussed. To argue that remagnetization is restricted to the LTC and ITC and that the HTC is of primary origin, additional experiments might be required. I would suggest the authors to conduct low-temperature susceptibility or magnetization experiments to see if Verwey transition can be identified. Verwey transition at 120 K is usually smeared in remagnetized carbonate, which is dominated by oxidized authigenic magnetite (Jackson and Swanson-Hysell, 2012). If the magnetic carrier of the HTC is detrital (titano)magnetite or biogenic magnetite, which can carry a primary remanence, Verwey transition should be detected.
Response 2: We agree that the wasp-waisted curves, indicative of SP grains mixed with larger grains, move close to but not within the so called remagnetization field of carbonates. We have emphasized that SP ferric oxides from weathering can induce similar behavior, as has been documented in carbonates from the Umbrian Apennines of Italy and accreted limestones in California. We have added a citation to the work on the Umbrian red/pink limestones (Channell and McCabe, 1994). Also, low temperature tests in the context of our work are ambiguous.